1. Field of the Invention
This invention relates to a light-emitting diode device designed to suppress thermal time-constant effects.
2. Description of the Prior Art
A light-emitting diode (usually abbreviated to LED) is one of the most widely employed components in optical-fiber transmission systems. This component in fact exhibits a linear current-power response characteristic and modulation of optical power by variation of the current can be obtained over a broad frequency range, the high frequencies being easily in the vicinity of 150 to 200 MHz under present technological conditions.
Transmission systems or optical buses usually employ a plurality of light-emitting diode (LED) transmitters as well as a plurality of optical receivers, each receiver being equipped with a PIN-photodiode or with an avalanche photodiode.
A few properties of optical transmitting and receiving devices will first be recalled in order to facilitate the following description and to gain a more complete understanding of the general object of the invention.
In the case of an analog transmission, the light-emitting diode (hereinafter designated as LED) is forward-biased with a current Io corresponding to the mean value of the modulation zone traversed on the current-power characteristic. Modulation is performed by varying the current and therefore the power on each side of this mean value.
In the case of digital transmission, it is possible to perform a modulation with respect to zero. For example, I=0 and P=0 represent the logical "zero" and I=I1 and P=P1 represent the logical "one". However, in the case of most LEDs, an all-or-none modulation produces a poor pulse response and it is preferable to perform a modulation with two levels, which is equivalent to biasing at the mean current value Io. This mean value is advantageously employed in the frequent cases of three-level coding of the transmission in order to define the third level or intermediate level, the other two being those which correspond to state "one" and to state "zero".
After optical-fiber transmission, the power is demodulated by means of a photodiode. This photodiode is either of the PIN type or of the avalanche type. In both cases, the optical noise depends on the value of the mean modulation power.
In order to obtain the highest signal-to-noise ratio, it is necessary to produce maximum modulation of the LED and to minimize the mean optical power.
If the light which arrives at the receiver comes from a single transmitter, the ratio between these two parameters which defines the modulation ratio is constant and within the range of 0 to 100% (a typical value being 50%). In respect of a fixed value of the modulation ratio, the signal-to-noise ratio increases as the square root of the mean power.
If the light arriving at the receiver comes from a plurality n of transmitters (as is the case with optical buses), the signal-to-noise ratio is given by a relation which employs ##EQU1## where .DELTA.Pi is the modulation of the transmitter i and Poi is the mean power emitted by the transmitter i. The modulation of the transmitter is thus liable to be masked by the noise produced by all the transmitters 1 to n. The degradation of the dynamic range is proportional to .sqroot.n in the case of powers which are considered equal since in this case, the ratio may be written ##EQU2##
The electric power PE=I.times.VD dissipated by a LED depends on the current I injected into the diode and also on its voltage drop VD. This power, which is dissipated at the level of the junction, produces a temperature rise which in turn causes a drop in the emitted optical power. The junction temperature TJ depends on the electric power PE, on the ambient temperature TA, on the junction-ambient heat resistance Rth.sub.J-A and can be expressed in the form: EQU Tj=PE.times.Rth.sub.J-A +TA
If the electric power applied to the diode varies as a function of time, the junction temperature follows with a time constant which can be defined by the product
Rth.sub.J-A .multidot.Cth.sub.J-A
where Cth.sub.J-A represents the junction-ambient heat capacity.
A variation in the emitted optical power corresponds to the aforementioned time-dependent variation in the junction temperature and is an undesirable parasitic effect which interferes with the modulation of the diode.